Microseismic measurements can be characterized as a variant of seismics. In conventional seismic explorations a seismic source placed at a predetermined location, such as one or more airguns, vibrators or explosives, is activated and generates sufficient acoustic energy to cause acoustic waves to travel through the Earth. Reflected or refracted parts of this energy are then recorded by seismic receivers such as hydrophones and geophones.
In passive seismic or microseismic monitoring there is no actively controlled and triggered seismic source at a known location. The seismic energy is generated through so-called microseismic events caused by subterranean shifts and changes occurring within the subterranean formation and which at least partially give rise to acoustic waves which in turn can be recorded using suitable receivers. Microseismic events may have a man-made cause, such as hydraulic fracturing or may have an entirely natural origin. Although the microseismic events may be a consequence of human activity disturbing the subterranean rock, they occur without any active seismic source device which is operated to emit an acoustic signal. Microseismics may be used in the hydrocarbon industry to investigate petrochemical properties of the subsurface, monitor production of hydrocarbons from a reservoir, monitor fracturing/stimulation of the subsurface and/or the like. In a microseismic survey, passive seismic data is acquired and then processed to determine the properties of a subterranean section of the earth.
In a microseismic survey, a plurality of seismic sensors are deployed at positions proximate to the Earth's surface, in deep wells and/or in shallow wells. In multicomponent microseismic surveys, “three component” sensors are used to acquire the microseismic data. Three component sensors include three or more, typically mutually orthogonally oriented particle motion sensing elements. The sensing elements may be geophones, but may also be accelerometers or any other sensing device known in the art that is responsive to velocity, acceleration or motion of the particles of the Earth proximate the sensor. The seismic sensors generate electrical and/or optical signals in response to the detected particle motion or acceleration along each of the three directionally sensitive components. The acquired signals may be used to create a time-indexed recording of the signals from each sensor and this microseismic data may be processed to determine properties of a subsection of the Earth, fracturing operation in the subsection of the Earth, properties of a reservoir producing hydrocarbons and/or the like.
Noise filtering is important in microseismic data processing because passive seismic interpretation is very sensitive to data quality. In microseismics, the uncertainty of the estimated microseismic event locations is strongly dependent on the data quality. As such, the uncertainty in the location of the microseismic events can be reduced by improving the signal-to-noise ratio of the processed seismic data.